This interim report on a controlled therapeutic trial of azathioprine in ulcerative colitis deals with the first 40 patients to complete a one-year period of maintenance treatment with azathioprine or with dummy tablets. The patients all suffered from classical ulcerative colitis and were in an actual attack of the disease at the time of admission. The attack was treated with a standard corticosteroid regimen and the patients were assigned at random to maintenance treatment with real or dummy azathioprine tablets, using a stratified design. The treatment and control groups were closely similar at the beginning of the trial.The effect of treatment has been assessed on the basis of the number of relapses of the disease occurring during the one-year trial period, supplemented by an assessment of the sigmoidoscopic picture and of the histological findings on serial rectal biopsy. In the patients receiving azathioprine the disease ran a more favourable course than in the control group. After the attack had been treated 11 of the 20 patients on azathioprine were symptom-free throughout the rest of the one-year trial period compared with only 5 out of 20 in the control group. The only three patients classed as failures were all in the control group. These differences just fail to reach conventional levels of statistical significance.Azathioprine is not dramatically successful but may still be a useful addition to the medical treatment of ulcerative colitis, particularly if conventional medical treatment is ineffective and there are reasons for wishing to avoid radical surgery. In the dose used azathioprine was virtually free from undesirable side effects.